Why so many student flats

Student numbers grew sharply up to about 2012 but have plateaued since visual.ons.gov.uk/how-has-the-student-population-changed/

As to why so many student flats:
1. It's partly the market catching up with demand; there has been a shortage of accommodation in many places. 
2. Some shared houses previously made available to students have either been turned into flats for sale or redirected at the immigrant market. 
3. The bank of mum & dad.
4. Student flats are particularly popular with overseas students especially from places like China (whose parents may also be buying).
5. Heavily marketed BTL schemes, some with "guaranteed" returns.

I would be wary of the sector at the moment, because UK student demand fell this year and overseas student numbers could fall sharply - partly because of Brexit and partly because of hostility to immigration generally.

What's wrong with a mattress and a bucket? 

Council trying to keep students in one place - easier to monitor re noise etc

I've seen it suggested, perhaps on another thread here, that a lot of students don't like these self contained student flats, preferring house shares or HMOs for social reasons. Too easy for the less outgoing students to stay locked up in their rooms seeing no one. Certainly my daughter, currently at uni, found that. She did year one in a student flat but year two onwards has shared.

In my area students have predominantly tended to reside in shared terraced houses in the vicinity of the unis....there is a huge student population in the nearest village to Keele university even though it has it's own accommodation blocks (not enough obviously)
Now though there are student blocks going up everywhere.
The result of this,in my view,will be a lot of terraced houses vacant.... landlords will either have to target the non student population or sell them off which could have a knock on effect if there's suddenly a surplus on the market,ie prices will drop which will be a good thing for 1st time buyers.
Only time will tell but I reckon this bubble will burst sooner rather than later

Who's buying them? It appears that developers are building out then selling off to individual investors -- With "guaranteed rent" periods of 2-4yrs and yields of ~8% it's attractive to a number of "normal" people.

Who's living in them?  In my experience, these custom build student accommodation units are marketing for 2x the comparable market rate for 4-6 bed HMOs (House of Multiple Occupancy, the typical student house-share situation).  There have been a number of high-profile cases where the developer has gone back to planning and asked for change-of-use to service accommodation, as the buildings are standing empty.

I'd be interested in hearing from anyone that has information on the actual strategy being employed by these developers?

A couple of years ago we looked into buying one of these flats as an investment. The price was a steal and occupancy was guaranteed. It looked too good to be true and it was.

Delving into it further, we discovered that the local university did not recommend the flats to their students. I can't remember why. Where were the guaranteed occupants coming from? I'm not saying all schemes are like that. But careful research is needed.